Wave filter



2 Sheets-Sheet l INVENTOR.

ATTORNEY.

Oct. 25, 1938. L. F. CURTIS WAVE `FILTER Filed June 27, 1956 l ll/ oct. 25, 193s. L F. CURUS 2,134,175

' WAVE FILTER Filed June 27, 1956 2 Shecs-Sheec 2 25a-'g' i WQ@ ,4 @MAZ ATTORNEY.

Patented Oct. 25, 1938 UNITED STATES WAVE FILTER VLeslie Curtis, SpringeldM ass., assignorrto yUnited American Bosch Corporation, 'Springfield, Mass., a corporation Vof New ,York Application June 27, 1936, .SerialNo-. 87,711

4 claims. (c1. 17a-'44) This invention relates to radio apparatus and more particularly to a tuned filter circuit for eliminating an undesired frequency from a band of transmitted frequencies.

In the past, filters of various kinds have been employed in the antenna circuit of radio receiv. ers to minimize the effect of interfering stations and were commonly known as wave traps. `In the smaller types of superheterodyne receivers the antenna isr connected to the first detector stage and no intervening stage of tuned radio frequency is employed. In such receivers an interfering signal at or near the frequency to Whichthe intermediate frequency amplifier is tuned will be transmitted directly through the first detecto-r to the intermediate frequency amplifier. Since 465 kilocycles has become almost a standard tuning frequency for the intermediate amplifier, the interference is usually code transmission from stations handling ship to shore trac and operating on frequencies near 465 kilocycles. While filters according to the prior art practice reduced the interference from a particular station, the interfering signal was not eliminated and the reproduction of a desired program suffered accordingly.

It is therefore one of the objects of this invention to provide a lter circuit which shall be more effective in blocking a given transmitted frequency.

Another object is to provide a filter circuit which may be included in the receiver construction ormay bein the form of a separate unit.

Still another object is to provide a filter unit which is more stable in construction and requires less frequent adjustment.

A further object is to provide a filter unit which is more compact in structure and economical in assembly.

Other objects and advantages will in part be specifically stated and in part be obvious when the following specification is read in connection with the drawings, in which:

Fig. 1 is a schematic diagram of the filter; Fig. 2 is a top view'of the ilter assembly; Fig.,3 is a section taken on line 3-3'of Fig` 2; and Fig. 4 is a botom View of the filter assembly.

Referring in more detail to Fig. 1, the numeral I indicates any suitable antenna. A series resonant circuit comprising an inductance 2 and a variable condenser 3 is connected between the antenna and ground. A parallel resonant circuit comprising inductance 4 and variable condenser 5 is connected in series with the antenna I and to the antenna post of a receiver 6. An electrostatic 'shield'l may be used if necessaryto eliminate undesired electrostatic' coupling betweenthe inductance 2 or condenser :Sand the outputster` minals of the device.- The inductances. 2 and ,4 are magnetically .coupledland'on'e ofthe inductances is movablysupportedto provide a variable. coupling between.theinductances; aslwill ,later `be shown and explained in detail.

In Fig.2 there is showna top plan view of the component filter elements as, arranged .onfan y insulating base. The .reference numeralsA indicating the diagrammatic showing of. the parts'in Fig. 1 Awill be' applied tolike parts in Fig. Zand the followingiigures.

The assembly'ismounted:upon a base .8 whichv may be a disk of any suitable insulatingmaterial such as one of the synthetic resins. lAttached` to the base 8 by means of.` mounting bolts 9 and nuts l 0 is double alignment condenserofithetype commonly used in intermediate frequency.k transformer's. `The double alignment condensercomf prises a vceramic base" I Iv upon Vwhich is mounted compression-'type variable lcond'ens'ers 3 `and`l5 which may beadjusted by the compression screws I4 and I5 respectively. The condenserr 5 has terminals I6 and I 'I which project upwardly through slots in the ceramic base II, and the condenser 3 has similarly arranged terminals I8 and I9. A bow. spring 20 kof non-magnetic material is secured to the ceramic base II byv means of the mounting bolts 9 and extends transversely over the base II asbest seen in Fig. 3.

A longitudinal slot is provided in the bow spring y through which projects a tongue 2I on the coil' form supporting the inductance 4. Secured by a mounting bolt 9 and extending angularly from the ceramic base II is a coil bracket 22 of sheet `metal which supports the coil form 23 and inductance 2. The inductances 2 and 4 are each universal wound and in two sections as illustrated by the inductance 2 of Fig. 2.

In Fig. 3 is shown a cross section of the device taken on line 3--3 of Fig. 2. The movable inductance 4 is adjusted bythe screw r2li which is carried by a threaded opening inA the mounting strip 25. A metal shell 26 having a mounting bracket 2l is spun over and secured to the insu'- lating base disk 8.

Fig. 4 is a bottom View of the device showing the condenser adjusting screws I4and I5 accessible through an opening in the base disk 8. The mounting strip is secured by the screws 9 and extends transversely across the opening in the disk 8, carrying the centrally located coil adjusting screw 24.

In the operation of the device, the series circuit comprising inductance 2 and capacity 3 and the parallel circuit comprising inductance 4 and capacity 5 are both tuned approximately to the interfering frequency which is to be eliminated.- Even though the circuits are tuned exactly to the undesired frequency, there will be a residual voltage at this frequency due tothe radio lfrequency resistance of the inductances 2 and 4.

By suitable regulation of the mutual coupling between the inductances, the residual voltage in the respective circuits may be made to cancel out and the terminal voltage at the interferingfrequency will be reduced to substantially Zero.l

When the circuits are slightly off tune with the interference, the voltage thereof. may still be reduced to zero by the adjustment of coupling.

Inductances having a relatively poor Ypower fac' tor may be employed in this present device and. so reduce the expense of parts, whereas in the prior art devices their eifectiveness depended upon the reduction of the power factor to the lowest possible values. In tests it has been found that an interfering signal can be reduced on' the order of 30:1 or slightly better by the proper adjustment of coupling, over Ythe reduction possible with the same circuitsv without the mutual coupling vbetween inductances.

It will be obvious that many uses and special applications of the invention exist, other than the particular one illustrated and described. Likewise, many changes and` modifications may be made in the invention by any one skilled in the art without departing from the spiritand scope of the invention as set forth in the followof said frequency differing in phase from the residual voltage of said shunt impedance arm, and means for establishing a mutual coupling relation between said impedance arms, whereby said residual voltages may be substantially eliminated.

2. In a filter circuit for the attenuation of a predetermined frequency, a shunt impedance arm comprising a series resonant circuit tuned to said frequency'and having a residual Voltage of said frequency across the terminals thereof, a series impedance arm comprising a parallel resonantcircuit tuned to said frequency and having a residual ,voltage of said frequency differing in phase from the residual voltage of said shunt impedance arm, and means for establishing a mutual coupling relation between said impedance arms, whereby said residual voltages may be substantially eliminated.

3. In a lter circuit for the attenuation of a predetermined frequency, a shunt impedance arm tuned to said frequency and having a residual voltage of said frequency across the terminals thereof, a series impedance arm tuned to said frequency and having a residual voltage of said frequency differing in phase from the residual Voltage of said shunt impedance arm, and means for combining said residual voltages in one of said impedance arms, whereby they may be substantially eliminated.

4. In a filter circuit for the attenuation of a predetermined frequency, a shunt impedance arm tuned to said frequency and havinga residual voltage of said frequency across the terminals thereof, a seriesimpedance arm tuned to said frequency and having a residual voltage of said frequency differing in phase from theresidual Voltage of said shunt impedance arm, and means for impressing upon theshunt impedance arm the residual voltage existing across said series impedance arm, whereby the residual voltage may be substantially eliminated.

LESLIE F. CURTIS. 

